Heretofore interconnection between the various components of VLSI circuits has been implemented using either metal, metal silicide, polysilicon or a combination of these as a conductor. Metal, either in a pure form or as an alloy, is the most commonly used conductor in VLSI technology. Aluminum and aluminum alloys, because of a number of eminently desirable properties, are foremost among the materials used in integrated circuit metalization. Salient among the characteristics that render aluminum desirable are its ease of deposition, low contact resistance, high conductivity, good adherence and bondability. These physical characteristics are complemented by its availability and moderate cost as compared to arguably competing materials.
However, certain aluminum silicon interactions detract from aluminum's general desirability, especially in VLSI applications. A thorough explication of such phenomena is provided in an article by Pramanik and Saxena titled "VLSI Metalization Using Aluminum and its Alloys," appearing in the January and March, 1983 issues of Solid State Technology. In particular, aluminum metalization is typified by electromigration, hillock formation and dissolution of silicon into aluminum. These effects are exacerbated in the presence of the narrow dimensions and very shallow junctions encountered in VLSI processes. Furthermore, the high surface reflectivity presented by aluminum tends to result in poor line definition when aluminum conductors are patterned according to standard optical lithographic techniques.
The use of polysilicon in an attempt to circumvent these difficulties is less than entirely successful because polysilicon is a comparatively poor conductor even when doped with selected impurities. Also, with regard to the fabrication of CMOS devices, two separate masking layers are needed to selectively dope the disparate N and P channel structures. Additional masking layers are undesirable because they can be expected to limit process throughput. The modest conductivity of the doped polysilicon can be enhanced by depositing an appropriate capping metal, but this interconnect scheme is also plagued with higher than desired contact resistance.
Another approach is to use a metal silicide as the interconnect material. The silicide provides a somewhat improved conductivity, although significantly inferior to that of a pure metal. The reduced conductivity, as with the polysilicon system, could be improved by adding a capping metal layer. The concept of dual layers of metal and silicide has been proposed as an effective compromise, but other disadvantages attend. To wit: high contact resistance, stress, and process inconsistency render this approach undesirable.
What is desired, therefore, is an interconnect material of requisite conductivity that provides an effective barrier to diffusion, low contact resistance, and suppression of hillock formation and electromigration. The concept of dual layers of metal and silicide has been proposed as an effective compromise to the above concerns. However, this approach requires precise control of the silicide formation to avoid local excesses of either the metal or the silicon. Such excesses can result in high stress or high resistance, respectively.